Answer: For all our PMC modules other then those listed as PMC-DX modules, our PCI bus interface chip is an Altera FPGA and is powered by 3.3V. The Altera FPGA generates a Voltage Output High (Voh) of 2.4V minimum. This Voh of 2.4V is sufficient to call the board compliant with 5V PCI.
Since we do drive only to 3.3V we are also tolerant in 3.3V systems. However we do not use termination diodes as is required by the standard to be 3.3 PCI bus compliant. Because of this Acromag PMC modules should be able to operate without problem in both 5V and 3.3V backplanes. Currently the keying of the PMC modules allow plug into either 3.3 or 5V systems.
The Altera FPGA does have diodes available in the FPGA device. We could for larger project programmably enable the diodes. Once the diodes are enabled the board could only be used in a 3.3V system.
Acromag's PMC-DX board use a PLX bus interface that is compliant with both 5V and 3.3V.